Photosynthetic electron transport

chloroplast structure bound by

outer and inner envelope membrane enclosing the stroma

grana lamellae

internal membrane system composed of stacked thylakoids

stroma lamellae

unstacked connecting membranes

thylakoid lumen

interior space enclosed by membrane system

Chlorophyll is located

within thylakoid membranes

Light reactions

use absorbed solar light energy to oxidize water (releasing O2) and drive electron transport to synthesize energy-carrying molecules

light harvesting complexes

Antenna complexes consisting of pigment-protein complexes

Light Harvesting Complexes composed of

proteins and lipids and chromophores/pigments

PS II

Oxidizes water to generate electrons, protons (H+), and oxygen gas (O2)

PS I

Re-energizes electrons using light energy to reduce NADP+ to NADPH via ferredoxin and FNR

PSII and LHCII are spatially localized predominantly in the

stacked grana lamellae

PSI and ATP Synthase are localized in the

unstacked stroma lamellae

Cyto b6f Complex is equally distributed

between grana and stroma lamellae

Protons accumulate in the thylakoid lumen through water splitting by the ______ in PSII

Oxygen-Evolving Complex

Protons pumped/shuttled from the stroma into the lumen by

Plastoquinone and the Cytochrome b6f complex

elements of the photosynthetic electron transport chain

Photosystem II and Oxygen-evolving complex
Pheophytin
Plastoquinone
cyto b6f complex
plastocyanin
Photosystem I
ferredoxin
ferredoxin-NADP reductase

cyclic electron transport

alternative pathway where electrons excited at PSI pass to Fd but instead of reducing NADP+ to NADPH, are recycled back to the Cytochrome b6f complex / Plastoquinone pool

water:water cycle

Electrons derived from water splitting at PSII flow through the chain to PSI, where they are transferred to O2 (reducing it back to H2O)

water:water cycle function

Acts as an electron sink to prevent over-reduction of the ETC, dissipating excess energy and scavenging ROS when CO2 assimilation is limited

Non-Photochemical Quenching

Safely dissipates excess excitation energy as heat

State Transitions

Phosphorylation-driven re-organization of LHCII away from PSII toward PSI to rebalance excitation energy

Alternative electron pathways

Cyclic electron transport and the water-water cycle

Anatomical/morphological changes

altering leaf angles, leaf wax, or increasing trichome density to reflect light

adaptations in shade plants

thinner leaves, more total chlorophyll per reaction center, higher PSII to PSI ratio to maximize light capture under low light conditions.

adaptations in sun plants

Thicker leaves, less chlorophyll per reaction center, higher concentrations of Rubisco for increased CO2 assimilation capacity, larger pools of xanthophyll cycle components for NPQ heat dissipation under bright light

too much light or if electron transport between the reaction centers is blocked can lead to

over-reduced ETC, formation of triplet chlorophyll and ROS causing photooxidative stress and direct damage to photosystems

cyto b6f complex serves as the central redox to carry electrons from ___ to ____

PS II to PS I

b6f complex pumps 4 H+ into the thylakoid lumen for every 2 PQH2 oxidized generating the

proton motive force required for ATP synthesis

cytochrome b6f complex acts as the docking site where electrons from ______ are recycled during CET

Ferredoxin

Under uneven light conditions (or light over-saturating PSII), kinase enzymes

phosphorylate LHCII

Phosphorylation changes charges on the stromal surface, causing LHCII trimers to dissociate from

PSII in stacked grana lamellae and migrate to PSI in unstacked stroma lamellae

shift in light absorption capacity away from PSII toward PSI, maintains

balanced excitation energy between both photosystems

Plastoquinol

lipid-soluble carrier that carries electrons through the membrane to the cyto b6f complex

plastocyanin

water-soluble copper protein in the lumen that carries electrons from Cyt b6f to PSI

what do light harvesting comlexes do?

collect light photons across various wavelengths and transfer excitation energy to the reaction centers

herbicides

block electron flow by binding to PQ binding site on PSI

quenching

Thermal dissipation of excess energy in LHCII triggered by lumen acidification

D1 Protein Turnover

Rapidly degrading and replacing damaged D1 reaction center proteins

water-water cycle/chlororespiration

Safely transfers excess electrons to O2

Acidification of the lumen directly triggers ____ via activation of VDE in the Xanthophyll cycle.

energy-dependent non-photochemical quenching

photoinhibition

damage and degradation of the D1 protein core of PSII